The present invention relates to a process for flash spinning strong discontinuous fibers, especially strong discontinuous plexifilamentary film-fibril strands, from fibre-forming polyolefins in which a mixture of solvent and strands formed at the exit of a spinneret in a flash spinning process is subjected to a high velocity fluid. The fluid is a mixture of steam and water.
As used herein, "plexifilamentary film-fibril strands of polyolefin" means strands which are characterized as a three dimensional integral network of a multitude of thin, ribbon-like film-like elements of random length and of a thickness in the range of about 1-20 microns, with an average thickness of less than about 10 microns, generally coextensively aligned with the longitudinal axis of the strand. The film-fibril elements intermittently unite and separate at irregular intervals in various places throughout the length, width and thickness of the strand to form the three-dimensional network. Such strands are known, being described in further detail in Blades and White, U.S. Pat. No. 3,081,519 which issued Mar. 19, 1963. In addition, as used herein "discontinuous" means that the strands have a length of not more than 100 mm.
Blades and White describe a flash-spinning process for producing continuous plexifilamentary film-fibril strands from fibre-forming polymers. A solution of polymer in a liquid that is a non-solvent for the polymer at or below its normal boiling point, is extruded at a temperature above the normal boiling point of the liquid and at autogenous or higher pressure through a spinneret into a medium of lower temperature and substantially lower pressure. This flash spinning causes the liquid to vaporize and form a continuous plexifilamentary film-fibril strand. Preferred polymers include crystalline polyhydrocarbons e.g. polyethylene and polypropylene. Commercial spunbonded products have been made from polyethylene plexifilamentary film-fibril strands obtained by flash-spinning using trichlorofluoromethane as solvent, but that halocarbon has been implicated in the depletion of the earth's ozone.
It is possible to form discrete fibers by spinning a polymer of relatively low molecular weight in a conventional spinning process, which is how some commercial pulps are manufactured, but it is more difficult to produce discrete fibers from high molecular weight polymers. It is known that the spinneret and tunnel used in flash spinning a polymer solution are important with respect to properties of flash spun continuous fibers, e.g. tenacity and elongation to break. For instance, U.S. Pat. No. 4,352,650 of Marshall, which issued Oct. 5, 1982, discusses optimization of the tunnel for increasing fibre tenacity e.g. from 4.2 to 5.2 grams/denier, of flash spun continuous fibers. In general, fibre tenacity may be increased by as much as 1.3 to 1.7 times by using a tunnel at the exit to the spinneret.
Various methods are known for making discrete fibers using a secondary fluid. For example, flash-spinning a polyolefin discrete fibre from polymer dissolved in solvent containing water in quantities sufficient to form an emulsion or inverse emulsion is described by Kozlowski in U.S. Pat. No. 4,054,625, which issued Oct. 18, 1977. A process for the manufacture of discontinuous fibers is described in published PCT patent application No. WO 93/06265 of A. Shah. U.S. Pat. No. 4,600,545 of P. Galli et al discloses ejecting a solution of a polyolefin at above autogenous pressure into a zone of lower pressure and, after allowing partial expansion of the solution, contacting with a jet of a. high-speed fluid.
U.S. Pat. No. 5,043,108 of S. Samuels, which issued Aug. 27, 1991 discloses flash spinning of a mixture of organic solvent, polyethylene and a non-solvent or spin aid, especially water or an alcohol, or mixtures thereof, in which the amount of water is less than the saturation limit of water in the organic solvent. A process for the manufacture of a polyolefin pulp in which strands are formed and shredded is disclosed in U.S. Pat. No. 5,093,060 of S. Samuels and V. G. Zboril, issued Mar. 3, 1992. However, while it is possible to produce continuous filaments using a flash spinning process and to subsequently shred the continuous filaments by mechanical means to form discontinuous filaments, such mechanical shredding tends to fuse the ends of the shredded filaments, even if the shredding is carried out under water. Fused ends make it difficult or impossible to open up the resultant web of fibers, as well as reducing fibre orientation and strength. For these reasons, processes that produce discontinuous fibers without requiring use of mechanical shredding means would be preferred.